The authorial intent, as suggested by the verse's surrounding context, seems to be that of instilling Christ-like meekness and humility (Matthew 11:29) in the hearts of boastful people:
1 Clement 30 For God, he saith, resisteth the proud, but giveth grace unto the humble. Let us clothe ourselves with concord, being humble, temperate, keeping ourselves far from all whispering and evil speaking, justified by our deeds, and not by our words. For he saith, He who saith many things shall, in return, hear many things. Doth he that is eloquent think himself to be just ? Doth he that is born of woman and liveth but for a short time think himself to be blessed? Be not abundant in speech. Let our praise be in God, and not for ourselves, for God hateth the self-praisers. Let the testimony of right actions be given us from others, even as it was given to our fathers who were just. Audacity, self-will, and boldness belong to them who are accursed of God; but moderation, humility, and meekness, to them that are blessed of God.
Doth he that is born of woman and liveth but for a short time think himself to be blessed ?
serves as a stern reminder of our own mortality; if we perish like flies (Isaiah 40:6), then what exactly is there to be boastful of in the first place ?
Be not abundant in speech.
the context, as noted above, points to this as referring to self-congratulatory or self-praising speech.
There is also a stylistic note to be added, in that the ancient Christian author is somewhat ironically contrasting the brevity of human life, with the length of the self-adulatory tirades in which people characterized by such self-centered tendencies quite often indulge in.